docs: def iterate_pubmed_identifiers(graph) -> Iterable[str]:
      """Iterate over all PubMed identifiers in a graph.
  
      :param pybel.BELGraph graph: A BEL graph
      :return: An iterator over the PubMed identifiers in the graph
      """
      return (
          data[CITATION][CITATION_REFERENCE].strip()
          for _, _, data in graph.edges(data=True)
          if has_pubmed(data)
      )